Magnetic resonance imaging ("MRI") is a highly useful technique for diagnosing abnormalities in biological tissue. Medical MRI scanning requires creation of a substantially constant "primary" magnetic field, which passes through a patient's body. Additional, linear time varying "gradient" magnetic fields are typically superimposed on the primary field, based on the desired scanning sequence. The patient is also exposed to electromagnetic waves in the radio frequency range, which also vary with time in particular patterns. Under the influence of the magnetic fields and the radio waves, certain atomic nuclei within the patient's tissues resonate and emit other radio frequency waves. By known mathematical techniques involving correlation of the magnetic field patterns applied at various times with the radio frequency waves emitted by the patient, it is possible to determine physical conditions at various locations within the patient's body. This information is typically displayed as an image with intensity corresponding to the concentration and/or physical state of certain nuclei of interest. The concentrations or physical states of different substances ordinarily differ for differing kinds of tissues within the body, and also permit the physician to see abnormalities, such as tumors.
A magnetic resonance imaging ("MRI") device is a highly complex, sensitive system. A typical MRI device comprises a magnet with an imaging volume for exposing a patient to a static magnetic field, a gradient coil system for establishing the linear, time varying gradient magnetic fields in three dimensions necessary for a particular scanning sequence and shimming coils for cancelling non-uniformities in the magnetic field. A transmitting coil for transmitting radio frequency pulses to the patient during the scanning sequence and a receiving coil for picking up magnetic resonance imaging data during an actual scan, are provided as well. The same coil can act as both the transmitting and receiving coil. The receiving coil is typically coupled to a variable amplifier, which amplifies the received data to an adequate level for further processing. A frequency down converter is typically provided to shift the amplified signals, which are in a high frequency range, to a lower frequency range suitable for analog-to-digital ("A/D") conversion by an A/D converting array. The A/D converting array is coupled to a digital data processor, which filters and processes the data. The data processor provides the processed data to a computer, which further processes the data for display by an image display system. A controller typically provides synchronization pulses to the major subsystems of the MRI device to coordinate their operations.
Deviations from proper performance can arise in any of these subsystems or components due to component degradation, drift of analog components, environmental fluctuations, magnetic drift, and system or component failure, for example.
Various techniques are known for detecting and diagnosing variations from normal system or component operation. One diagnostic program for evaluating an MRI device developed by Fonar Corporation "Fonar"), assignee of the present invention, is referred to as MSSR, or multi-slice scan reconstruction. In MSSR, digital data representative of raw magnetic resonance imaging data of a phantom object, such as a cube filled with nickel chloride, is calculated. The calculated data is processed by the MRI device and an image derived from the data is displayed. The displayed test image is viewed for errors, such as image artifacts, to identify problems in the display or data processor of the MRI device. Image artifacts indicative of a problem include banding, multiple images or fuzziness, referred to as ghosting.
Another diagnostic program developed and used by Fonar is incorporated into Fonar's ULTIMATE.TM. scanner. Raw digital image data from an actual scan of a phantom object is loaded into memory and the data is processed for viewing. Again, the data processing and display systems can be evaluated. A fixed frequency can also be injected into the receiver coil and a scan performed. The resulting image should be a distinct dot. Ghosting in the image indicates a temporal instability somewhere in the system.
No known diagnostic system can evaluate all the major systems of an MRI device. In addition, no known diagnostic system can evaluate the timing accuracy of a scanning sequence in an MRI device.